Canon EF 2x III Extender Review

The Canon EF 2x III Extender mounts between your Canon EOS Digital SLR and a compatible lens
(primarily Canon L Series lenses), extending the focal length of the lens by a very noticeable 2x.
The huge focal length increase of course comes with some penalties, including a modest financial cost, some size and weight addition and
more impacting, a decrease in image quality and maximum aperture.

If you have read the Canon 1.4x EF Extender III Review, you will find the Canon EF 2x III Extender review familiar.
The 2x III is similar to the 1.4x III and these reviews were prepared simultaneously.

The primary reason to use any extender is of course to gain a longer focal length/narrower angle of view from a lens – to frame the subject more tightly.
Here is an example of the focal length increase provided by the 1.4x and 2x extenders.

The above images (of a Christmas Cactus flower) were taken from the same tripod-mounted position (using the tripod ring) with a
Canon EF 180mm f/3.5L USM Macro Lens on a
Canon EOS 5D Mark II camera body.
The difference in framing (angle of view) is quite noticeable between the three examples shown.
This is the lone, but very significant, advantage provided by extenders.

A significant disadvantage brought by the use of a 2x extender (any of brand or model of them) is that your lens' max aperture setting
is reduced/narrowed by a 2 stops – allowing at most 1/4 as much light into the exposure.
The lens aperture still opens to the same physical diameter, but the ratio of the aperture opening to the focal length is reduced – by 2 stops.
So, pick your lens' maximum aperture opening from the following list and understand that it will shift two values to the right
when any 2x extender is attached: f/2.0, f/2.8, f4.0, f/5.6, f/8.0, f/11, f/16.

DSLR cameras will continue to calculate auto exposures properly when used with Canon Extenders.

Since many Canon EOS camera bodies need a max aperture of f/5.6 or wider to autofocus, losing 2 stops of max aperture pushes f/4 and narrower lenses past their ability to autofocus.
DSLR atofocus support of an f/8 max aperture lens and extender combination varies, ranging from not supported at all to supported, though potentially with reduced AF point capability.
Some of the extender-compatible lenses start with an f/5.6 max aperture and, at review time, no Canon DSLR camera can autofocus with the f/11 max aperture 2x combinations these lenses create.
The lens and extender combo can always be used in manual focus mode, but a narrow max aperture (f/11 for the f/5.6 lenses)
creates a very dark viewfinder that makes manual focusing difficult.
Often better (when available) is to use Live View AF, which is generally supported with f/11 max aperture lens combinations.

Extenders do not affect the MFD (Minimum Focus Distance) of the lens they are mounted behind,
so the MM (Maximum Magnification) of the lens is also multiplied by 2x.
Do you need more macro magnification from your Canon EF 180mm f/3.5L Lens?
Add the Canon EF 2x III Extender to the setup for a 360mm 2:1 (2x) Macro lens.

Shooting with the Canon EF 2x III Extender mounted results in reduction of autofocus speed.
According to Chuck Westfall (Canon USA):
"As with previous EF Extenders, usage of Series III EF Extenders lowers AF drive speed to improve AF performance.
When Extender EF 1.4X III is used, AF drive speed is reduced by 50%.
When Extender EF 2X III is used, AF drive speed is reduced by 75%.
This may seem like a drawback, but in reality subject tracking performance remains quite high when Series III Extenders are used with IS II lenses.
This is due to improvements in AF precision made possible by the new microcomputer in the extenders."

Canon's Rudy Winston completes this story:

"Because AF systems are essentially computer-controlled to read and react to focus distance changes, the information must be modified so that the focusing movement (or sensitivity) compensates for the added presence of the extender. In the Canon EOS system, this is done by deliberately reducing drive speed when an extender is detected.

Before you immediately conclude that this is a problem, understand that this reduction in drive speed now corresponds to the effective speed you would achieve with the same EF lens alone. It compensates, automatically, for the reduced distance lens elements in the lens’s focusing group(s) need to move to refocus on a subject, with either EF Extender in place. Accordingly, overall AF performance remains essentially unchanged with an EF Extender attached, versus the lens’s AF speed without an extender."

With good lenses mounted on a 1-Series body and high-contrast subjects selected for AF, I'm not finding the 2x III AF speed a problem.
If you are tracking birds in flight, fast sports action at relatively short distances or shooting low contrast subjects in low light, expect a noticeable AF performance decrease.

The biggest downside to using a 2x extender, in my opinion, is the image quality reduction it causes.
Because the Canon EF 2x III Extender is multiplying the aberrations/flaws of the lens it is mounted behind by 2x and because there are now 9
additional lenses for light to pass through, reduced image quality should be expected.

The image quality of lenses varies from model to model; and therefore, the optical results of the various with-extender combinations also vary.
A great lens will typically produce the best image quality when used with the 2x and
the best way to see how specific extender+lens combinations perform is by using the
ISO 12233 Resolution Chart Tool.
All current extender-compatible lenses have ISO 12233 chart results from the with-extender combinations available (select the longest with-extender focal length value).
Compare the native maximum focal length to the extended focal length to see the optical change from the selected combination.

The Canon EF 2x III Extender reduces contrast and sharpness
(use the mouseover feature) of even the best lenses with wide open apertures.
Stopping the lens down one to two stops
(again, use the mouseover feature) usually improves the image quality of both the bare lens and the extended lens combination.
I'm much happier with 2-stops-down 2x III results than wide open results.

The 2x III adds a small amount of barrel distortion to the lens it is being used with.
Since many telephoto zoom lenses have pincushion distortion at their longest focal lengths, the added barrel distortion can
counter the pincushion distortion to a small extent.
Note that barrel distortion causes the center portion of the image (top image in the ISO 12233 tool) to be slightly enlarged,
which can make it appear to resolve more detail.

The Canon 180 L Macro is a lens that performs very well with extenders.
Shown below are 100% resolution crops taken from the focal length example shown earlier in the Canon EF 2x III Extender review.
These results show what a very sharp lens, stopped down to f/11 and used with good lighting can produce with extenders.

Each row below the image represents either the bare lens or the lens with the specified extender mounted behind it.
In each row, the left-most mouseover label is the native resolution of the combination and
the values to the right show Photoshop CS5 up-rezzed (resized larger) images created from the native image.

It is obvious that extenders produce better results than simply increasing the size of an image in Photoshop
(and this is of course only true when you do not want the entire native lens' FOV).
Also impressive is the resulting image quality in these extender examples.
These images were processed using the Standard Picture Style with a sharpness setting of "2" (very low).

As a rule, but subject to change, the Canon lenses compatible with the Canon EF 2x III Extender include fixed focal length L lenses with focal lengths of 135mm and longer,
zoom L lenses with at least 70mm of focal length on their wide end (the Canon EF 70-300mm f/4-5.6L IS USM Lens excluded).
Canon TS-E Tilt-Shift lenses are not included on Canon's official compatibility chart and do not have the 3 additional gold lens contacts for communicating extender information to the camera,
but as their image quality results shown on their reviews show, they can function with extenders.
The following list of discontinued lenses fitting these parameters are compatible, but may require camera microfocus calibration for accurate focusing (note that not all DSLR cameras support microfocus calibration):

Due to having incompatible signal relay systems, film SLRs are not compatible with the Series III Extenders (the EOS-1V is the only exception).

While some third party extenders *may* work properly with Canon's non-extender-compatible lenses, they probably will not report the reduction in aperture.
Attempting to mount a Canon extender on these same non-compatible lenses can damage the lens due to contact being made with the rear lens element.
Canon extenders simply do not fit behind the rear lens element on Canon's non-extender-compatible lenses.

The Canon extenders have a protective rubber ring around the front element to protect both the extender itself
and the lens it is being inserted into if your alignment is off when mounting the extender to a compatible lens.

The first task I undertook upon purchasing (retail) my 2x III was to add the with-extender-III ISO 12233 resolution chart results
to all of the current extender-compatible lenses.
These same lenses have results from the version II extenders and provide a lens-by-lens basis for comparisons between the two extender versions.
The series III extender results are the last set (with 1.4x III and with 2x III) of focal lengths available for each lens.
Here is the list of these lens reviews – which is also the comprehensive-at-review-time list of currently available extender compatible Canon lenses.
Always check the Lens Specifications and Measurements Tool for the most update-to-date compatibility specs.

The image quality differences most easily noticed between the 2x II and the 2x III are:

Results with the Extender 2x III are overall slightly sharper than the Extender 2x II with a difference being most noticeable in the mid-frame and corner areas.
The 2x II added pincushion distortion to the lens it was used with whereas the 2x version III adds slight barrel distortion.

Anomalous dispersion glass elements are used in the Series III extenders to reduce chromatic aberration and increase resolution and contrast.
The Extender 2x II did not have an issue with CA and neither does the III.

The Extender 2x III has Canon’s Super Spectra coating for reduced ghosting and flare.
The front and rear elements have Canon's fluorine anti-smear coating – making them very easy to clean.

Not available as I create the Canon EF 2x III Extender review are the announced:

Improvements in the Series III extenders promise to perform especially well on these specific lenses.

A microcomputer integrated into the Series III Extenders promises faster autofocusing and increased AF precision
when used with the Canon IS Supertelephoto Series II lenses mentioned above and (I expect) all future compatible lenses.
Better optical quality is also expected.

Note that "AF precision remains the same as the Series II Extenders when the Series III Extenders are used with earlier extender-compatible EF lenses." [Canon]
I did not notice any differences in this regard.

Note that Canon does not recommend stacking extenders.
While they do not fit together due to the lack of space at the rear elements,
an extension tube installed between them makes the fit possible.
But, Canon does not recommend using ETs at all due to reduced autofocus accuracy.
Image quality from stacked extenders will not be great.

Let's take a look at the Canon EF 2x III Extender mounted on a camera body.

The Canon EF 2x III Extender increases the physical length of any combination it is used
with by 2.08" (52.7mm) and increases the weight by 12 oz (325g).
The 2x III grew 2.6 oz (75g) heavier than, and is .21" (5.3mm) shorter than, the
Canon EF 2x Extender II it replaces.

Two additional lenses in the same number of groups (9/5 vs. 7/5) are found in the III (and likely contribute to the slightly increased weight).
The mounted extender also shifts the lens weight forward slightly for a potential change of balance.
The zoom and focus rings are of course shifted away from the camera body the same amount as the lens.
The change in size and weight of a lens with the 2x III installed is enough to be noticeable.

Below are the Series II and III Extenders shown for comparison.

Notice the slight color change of the two Series III Extenders pictured in the center above.
The III's new color matches Canon's latest white L lenses.

Like the 2x II, the Canon EF 2x III Extender is weather-sealed and very-solidly-built -
similar to Canon L Lenses in construction quality.
The Series III extenders include some durability improvements.

The lens mount pin and lens mount stopper pin are improved for higher endurance and,
as you can see below, there are now 7 (instead of 4) screws holding the rear lens mount to the body.

Considering that an extender can have nearly $20,000 worth of camera and lens attached to it, the extra strength insurance can be appreciated.
The lens mount release switch received a cosmetic upgrade, but functions the same.

At review time, the price differential between the still-available version II extenders and the version IIIs is rather high.
If the prices were equal, there would be no question that the III is the extender version to get.
But, saving money is one of the reasons to buy an extender instead of a lens with a longer native focal length.
And I'm sure that not everyone can justify the cost difference to go with the 2x III over the 2x II.
My personal opinion is that the 2x III's improvements are worth the difference in price, but we all value our images and money differently.

Of course, to get an extender at all is the first decision to make.
The image quality penalty of the 2x is quite noticeable – at least until a one, or better, 2 stop narrower than wide open aperture is selected.
Use the ISO 12233 chart results from the lens you are interested in using the 2x with – and decide for yourself if the resulting image quality is acceptable to you.
If it is, the resulting 2x focal length is very impressive.

My extender purchase advice is similar to the advice I gave in the Canon 1.4x EF Extender III Review.
Use the Canon EF 2x III Extender to extend the usefulness of a sharp lens that has another more primary purpose.
Buy a lens with the native focal length you need for best results.
Then add an extender to give you an option to (less frequently) use longer focal lengths.
Said another way, you generally will get better results from a 400mm lens than with a 200mm lens plus the Canon EF 2x III Extender,
but being able to shoot at a second focal length is a nice option.

Another good reason to use an extender is to create a longer focal length lens than is available natively.
For example, a Canon EF 800mm f/5.6L IS USM Lens
plus a 2x results in a not-natively-available 1600mm f/11 IS USM (MF-Only) Lens.

Buying an extender over a longer lens for financial reasons can also make sense.
The very long focal length lenses are considerably more expensive than extenders.

Using extenders to save space when traveling or hiking can be a good decision.

What types of photography are extenders used for?
Sports and wildlife are two of the most common photography pursuits utilizing extenders.
Photojournalism, law enforcement and many other types of photography can also make good use of them.

I don't use extenders a big percentage of the time, but I often have a 1.4x with me.
The Canon EF 2x III Extender does not go with me nearly as frequently, but I occasionally take it along too.
The Canon EF 2x III Extender significantly extends the reach of your sharp, compatible Canon lenses for a relatively low size, weight, and financial cost -
but it comes with an image quality penalty.

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